An inverter, or the like, which controls a motor switches a current supply path flowing from a power source to the coils of the motor, by controlling a switching device, thereby controlling the driving of the motor. Furthermore, by controlling the switching device, a transformer, or the like, adjusts the amount of current supplied from the power source to the reactor (coil), and the voltage generated by the power source is converted to an arbitrary voltage and output.
As a specific configuration of the switching device, a first switch element and a second switch element are connected in series, and furthermore, the contact point of the first switch element and the second switch element is connected to a reactor, which is an output section. In order to smoothen variations in the power source in the first switch element and the second switch element which are connected in series, a power storage element is connected in parallel, and driving of the motor is controlled and current is supplied by controlling the switching device. In order to discharge the energy that has been collected in the power storage element when power storage is not required, a discharge device is required.
A method in which a discharge resistor is connected to the power storage element is given as a discharge device of this kind. However, constantly passing current through the discharge resistor leads to the generation of heat in the resistor, and decline in the efficiency of the inverter. Therefore, a method is adopted in which the switch elements, and the like, are connected in series with the discharge resistor, and discharge is started by passing current through the switch elements by a discharge signal that is output only when discharge is required. Furthermore, a method for determining whether or not to output a discharge signal by detecting variation in the voltage between the two ends of a capacitor by a differential circuit has been proposed (see, for example, Patent Document 1).